Rheostat



April 1957 D. T. SIEGEL 2,790,882

RHEOSTAT Filed Oct. 7, 1954 ji J35 flu- 5 J0 INVENTOR.

United States Patent RHEOSTAT David T. Siege], Glencoe, Ell.

Application October 7, 1954, Serial No. 460,991

18 Claims. (Cl. 201-48) This invention is concerned generally with a rheostat, and more particularly with a small, wire-wound rheostat of improved operating characteristics.

The invention is hereinafter shown and described particularly with regard to a series or current limiting type, but it is to be understood that the term rheostat should be construed in its generic sense as including other types of devices having resistance elements and movable contacts thereon, such resistance elements usually being of arcuate configuration. Such devices are also used for voltage divider or potentiometer types, but are generi cally the same as the series or current limiting type noted earlier.

Sliding contacts obviously are subject to wear. Furthermore, they are subject to oxidation and other forms of corrosion, and also to deposits of dirt and other nonconducting particles. Hence, there is 'a tendency for the resistance of the sliding contact to vary with age. This gives rise to unpredictable characteristics of the rheostat, and of course cannot be tolerated. Furthermore, no backlash can be tolerated. Often the control knobs or the like of rheostats are mounted on the front of a panel and have indicia thereon alignable with indicia on the :panel for adjusting the rheostat. When backlash is present, the actual position of the rheostat slider may vary substantially from the position of the control knob or the like, and the variation will be non-constant, depending on the direction of rotation as well as on the speed of rotation.

Various means and methods have been adopted in the past to produce rheostats having good contact between the sliding contactor and the resistance element, and to preclude backlash. However, such means and methods generally have been expensive, and have required close manufacturing tolerances.

Accordingly, it is an object of this invention to provide a rheostat which is less expensive to produce and which .canbe manufactured with larger tolerances, both with regard to part dimensions and with regard to assembly techniques.

Another object of this invention is to provide a resilient sliding contact of improved construction which inherently cooperates with supporting structure in such a manner as to eliminate backlash.

More specifically, it is an object of this invention to provide an improved rheostat contacting member of resilient construction which wedges itself into place to eliminate backlash.

Yet another object of this invention is to provide a rheostat having a resilient spring contact of improved construction which simultaneously wedges itself into position to eliminate backlash and forces an electrically conductive member into flat surface engagement with the resistance element to retain good contact and uniform electrical characteristics throughout the service life of the rheostat.

Other and further objects and advantages of the pres 'ice 2 ent invention will be apparent from the following de scription wherein:

Fig. 1 is a side view of a rheostat constructed in ac-' cordance with the principles of my invention;

Fig. 2 is a rear end view thereof;

Fig. 3 is an axial sectional View taken substantially along the line 3-3 of Fig. 2;

Fig. 4 is a transverse sectional View taken substantially along line 4-4 of Fig. 3;

Fig. 5 is an exploded perspective view of the operating parts of the rheostat; and

Fig. 6 is an elevational view of the spring contact and the member by which it is carried.

Referring now in greater particularity to the figures, there will be seen a rheostat generally indicated by the numeral 10. The rheostat includes a ceramic base 12, said base including a generally fiat front disc 14 and a cylinder 16 extending rearwardly therefrom. The cylinder is provided with a pair of diametrically opposed grooves 18 accompanied by corresponding external ridges 20. A rib 22 extends laterally from the cylinder for reinforcing the cylinder and also to serve as a support for an electrical connector. An internal groove 24 is formed in the cylinder 16 and extends partially into the rib 22.

An arcuate insulating base 26 is formed of ceramic or other suitable material and is just short of a complete circle in circumference. Resistance wire 28 is wound on the core 26 and extends substantially from end to end thereof. Sheet metal bands 30 encircle the wire and core at the ends thereof and provide connecting tabs or lugs 32 extending generally radially from the core and wire. The core 26, wire 28, and bands 30 including the tabs 32 coact to form a resistance unit or element hereinafter identified by the numeral 34. This resistance unit or element is recessed into a groove in the back of the disc 14 of the ceramic base 12, and a coating 36 of enamel or the like is placed over the element and is baked on to insulate the element and to hold the element on the ceramic base, it being understood that some of the enamel or some other type of cement is interposed between the resistance element and the disc 14. The outer end 38 of the resistance element is not coated, either being left uncoated initially, or having the coating re moved therefrom.

A flat metal ring 40 (Figs. 3, 4 and 5) is slipped into the ceramic cylinder 16. This ring is provided with diametrically spaced cars 42 projecting radially outwardly into the grooves 18. The ring 40 further is provided with a generally axially extending strip 44 integral with the outer edge of the ring. The outer end of the strip 44 is bent over to form a finger 46 which extends into the groove 24 of the ceramic cylinder 16. The finger 46 and cars 42 block the ring 40 against rotation relative to the ceramic base 12 and provide the locking parts at positions spaced sufiiciently far apart to preclude fracturing the ceramic base. The ring further will be seen to have a struck out tooth 48 adjacent its inner edge.

A mounting sleeve 50 having external threads thereon is slipped through the ceramic cylinder 16, through the ring 40, and through a bore 52 in the disc 14 and aligned with the foregoing parts. This sleeve 50 is provided with a circumferential, radially extending flange 54 fitting against the face of the ring and having a notch 56 therein receiving the struck out car 48 of the ring, whereby to lock the sleeve 50 relative to the ring, and hence relative to the ceramic base 12. A nut 58 is threaded on the outer end of the sleeve 50 to clamp the sleeve in place on the base 12.

The rheostat 10 further comprises an additional ring 60 of larger diameter and having a radially extending tab 62 of substantial length. The ring 60 and tab 62 are formed of a material having low electrical resistancc, such cooper or bronze, and are mounted with the ring abutting the end of the ceramic cylinder 16 and the tab 62 extending out over the rib 22. The ring and tab are secured in this position by means such as a hollow rivet 64 extending through a hole in the tab and through an aligned bore 66 in the rib 22, a counterbore 68 being provided to accommodate the end of the rivet.

The rheostat further includes a shaft 70 which is preassembled with several other moving parts hereinafter to be set forth. The shaft 70 is provided with a flat 72 adjacent its outer end for receipt of a knob (not shown) In accordance with conventional practice, and further is provided near the flat with a circumferential groove 74 for receiving a split ring fastener 76. The shaft 70 is provided intermediate its ends with a yoke 78 welded to the surface of the shaft and having a radially extending tongue 80. This yoke is spaced from the ring receiving groove 74 a distance equal to the length of the sleeve 50 as will be apparent with reference to Fig. 3.

The rear or inner end of the shaft 70 is preassernbled with a knob 82 formed of plastic of suitable insulating properties, or of ceramic. The knob 82 includes a hollow cylindrical shank 84 of proper diameter snugly to receive the shaft 70. The knob further includes a head 86 having opposed flat sides 88. The rear end of the head 86 also is fiat as at 90, and is beveled or chamfered as at 92 into sides 88. The rear end of the head is provided with a shoulder 94, and the material above the shoulder likewise is beveled or chamfered into the sides as indicated at 96.

The front end of the head 86 is substantially wedgeshaped, having long diagonals 98 leading to a fiat nose portion 100. The under side of the front portion of the head is of generally the same outline as the upper portion, but is scaled down successively to form a generally wedge-shaped chin 102 leading to the shank 84, and joining the shank at a longitudinally extending ridge 104 on the shank. There is a similar ridge 106 provided diametrically oppositely on the shank.

The bore 108 extending through the hollow shank 84 extends also through the head 86, and a pair of diametrically disposed slots or grooves 110 is provided in the head. The shaft 70 is slipped through the bore 108 to bring the yoke 78 up against the inner end of the shank 84, and the end of the shaft then is staked or swaged outwardly to form protuberances 112 (Fig. 3) extending into the slots or grooves 110 to lock the head and shank comprising the knob 82 onto the shaft. it is also contemplated that the knob could be cemented on the shaft, thereby obviating the necessity of staking the end of the shaft, and further allowing the knob to be positioned otherwise than by abutting the yoke 78. A contact carrying spring 114 is provided for association with the knob 82. This spring is formed from a sheet of flat spring stock and has an internal opening 116 of generally the same configuration as the head 86, but slightly shorter than the distance between the inside of the shoulder 94 and the front edge or nose 100. This opening fits snugly along the sides 86 of the head and is designed to fit diagonally across the head with the rear edge 118 lying adjacent the shoulder 94, and the generally wedge-shaped front edge of the hole 1120 hearing up against the similarly shaped chin 102 and wedging tightly into place.

Flexibility is imparted to the spring member 114 by a pair of parallel cuts 122. leading into the member 114 from the rear edge thereof and along the sides of the hole 116. Small circular holes 124 terminate the cuts 122 to prevent tearing of the material at the ends of the cuts.

An additional cut line 126 extends from near the rear edge of the spring contact carrying member 114 and spaced outwardly from the cut lines 122. The cut line 126 extends forwardly of the hole 116 in a generally V-shape and further imparts flexibility to the member 114. The ends of the cut line 126 terminate in small 4 round holes 128 to prevent tearing of the material at the ends of the cut line.

The outside configuration of the spring contact carrying member 114 may vary widely within certain practical limits, and conveniently is generally similar to the outline of the hole 116. The member 114 includes a forwardly extending tongue 128, and this tongue has a contact holding portion 130 of generally rectangular configuration. The portion 130 is provided with a turned over tip 132, and with side lugs 134 extending in the same direction as the tip 132. A struck out projection 136 extending in the same direction as the tip 132 and lugs 134 completes a rectangular contact holding area or cup, and a punched dimple 138 extends into the cup area to form a pivotal contact with a contact block 140.

The contact block 140 comprises a generally rectangular block of suitable conductive material such as graphite impregnated bronze and is received in the cup of the spring contact carrying member as noted heretofore. The block 140 has a stranded flexible connector or wire 142 cast into it, and the opposite end of this wire is affixed in a raised lug 144 of a slip ring 146. This slip ring may be made of bronze or other suitable material which is generally wear resistant and of high electrical conductivity. The lug or projection 144 preferably is provided with an opening (not shown) into which the wire 142 projects, the material of the lug then being staked or swaged inwardly as indicated at 148 compressively to trap the wire in the lug. The slip ring 146 is provided with a central aperture which is generally circular with the exception of a radially extending cutout 150 best seen in Fig. 5.

The knob 82 is preassembled with the shaft 70 as previously indicated. The spring contact carrying member 114 then is assembled with the knob 82 as shown in Fig. 6, the spring contact carrying member being fiat at this stage. A coil spring 152 is placed about the shank 84 of the knob, and the ring 146 is moved over the shaft and shank, the radial opening 150 providing clearance for the tongue on the yoke 78, and also interfitting rather loosely with the elongated ridge 104 on the shank of the knob. The contact block 140 is placed loosely in the cup of the contact carrying member 114, and the shaft 70 is moved through the ceramic cylinders 16 and the sleeve 50 to the position shown in Fig. 3, the ring 76 then being placed in the groove 74 to hold the parts together as illustrated in Figs. l-4.

In the assembled position, as shown in Fig. 3, the under face of the block 140 fits flat against the winding 28 of the resistance element 34. This flat surface contact is effected by the universal joint provided by the cup and the struck out protuberance 138. The central portion of the contact carrying member 114 is forced tightly against the under side of the chin 102, thereby tightly wedging the contact carrying member against the head 86 and precluding any backlash. The outer material of the contact carrying member flexes upwardly relative to the head (to the right in Fig. 3), such flexing readily being accomplished by virtue of the spring metal characteristics of the member 114 and further by virtue of the cut lines 122 and 126. The spring member thus has a deflection imparted to it which holds the contact block 140 tightly against the resistance element, and also holds the material at the front edge of the hole tightly against the under side of the chin 102 positively to provide a tight fit precluding any backlash.

The coil spring 152 holds the slip ring 146 tightly against the ring 60 to form a sliding electrical contact therewith. The slip ring 146 is turned by engagement of the ridge 104 in the radial hole portion 150. Rotation of the shaft 70 and knob 82 are limited by engagement of the tongue 80 with the axially projecting strip 44 of the ring 40, thereby allowing the contact block to be carried from one end to the other of the resistance element 34 without overrunning such ends.

It will be seen that the foregoing structure positively eliminates backlash, and allows production to be more economical and with larger tolerances, both as to parts and as to production methods. The chin 102 of the knob which wedges into place with the contact carrying member heretofore has been set forth as being substantially wedge-shaped. The shape might also be considered to be analogous to that of the prow of a ship, and it might also be considered to be pyramidal in nature.

It will be appreciated that it is the diagonal surfaces of the chin 102 and the diagonal edges 96 of the knob that actually engage and carry the resilient contact carrying member 114. This effects a true wedging support positively precluding backlash. It further will be appreciated that the resiliency of the member 114, coupled with the spring 152, renders the exact axial position of the knob on the shaft non-critical.

Various changes in structure will no doubt occur to those skilled in the art, and will be understood as forming a part of my invention insofar as they come within the spirit and scope of the appended claims.

I claim:

1. A rheostat comprising an insulating base, a resistance element arcuately arranged on said base, means for making an electrical connection to said resistance element, a shaft rotatably carried from said base substantially concentric with said resistance element, an insulating member carried by said shaft and having a wedge shaped under surface, movable contact means slidably engageable with said resistance element and including a resilient member having an edge generally complementary to the wedge-shaped under surface of said insulating member, engagement of said movable contact means with said resistance element resiliently wedging said edge against said surface to preclude backlash between said shaft and said movable contact means, and means for making an electrical connection to said movable contact means.

2. A rheostat as set forth in claim 1 wherein the wedgeshaped undersurface of the insulating member is substantially pyramidal.

3. A rheostat as set forth in claim 2 wherein the resistance element is provided with a substantially flat surface, and wherein the movable contact means further includes a contact member and a universal joint connection mounting said cont-act member on said resilient member for maintaining said contact member in substantially flat surface contact with said resistance element.

4. A rheostat comprising an insulating base, a resistance element arcuately arranged on said base, means for making an electrical connection to said resistance element, a shaft rotatably carried from said base substantially concentric with said resistance element, an insulating member carried by said shaft and having a wedgeshaped under surface, movable contact means slidably engageable with said resistance element and including a substantially flat resilient plate-like member having an opening therein with an edge generally complementary to the wedge-shaped under surface of said insulating member, engagement of said movable contact means with said resistance element resiliently wedging said edge against said surface to preclude backlash between said shaft and said movable contact means, and means for making an electrical connection to said movable contact means.

5. A rheostat as set forth in claim 4, wherein the movable contact means further includes a contact element slidably engageable with the resistance element, and wherein the substantially fiat plate-like member is provided with deflected portions forming a cup receiving said contact element, and wherein said plate is provided with a dimple intermediate said deflected sections providing a protuberance engaging said contact element and forming with said deflected sections a universal joint mounting said contact element.

6. A rheostat as set forth in claim 4 wherein the sub stantially flat, resilient plate-like member is provided with a cut line demarcating said plate-like member into a plurality of areas and promoting flexibility between adjacent areas. I

7. A rheostat as set forth in claim 6 wherein the flat plate-like member is provided with a plurality of sections deflected from the surface of the member and forming a mounting cup, the material of said member within said cup having a projection formed therein, and a contact element received in said cup and against said protuberance as a universal joint, said contact element being received in sliding engagement with said resistance element.

8. A rheostat comprising an insulating base, a resistance element arcuately arranged on said base, means for making an electrical connection to said resistance element, a shaft rotatably carried from said base substan tially concentric with said resistance element, an insulating member carried by said shaft and having a wedgeshaped undersurface and an upper supporting surface disposed substantially diagonally therefrom, movable contact means slidably engageable with said resistance element and including a resilient member having an aperture therein with an edge generally complementary to the wedge-shaped under surface of said insulating member, said resilient member being diagonally mounted on said insulating member and in wedging engagement with said under surface and the diagonally disposed upper surface, engagement of said movable contact means with said resistance element resiliently effecting such wedging to preclude backlash between said shaft and said movable contact means, and means for making an electrical connection to said movable contact means.

9. A rheostat as set forth in claim 8 wherein the resilient member comprises a substantially flat plate.

10. A rheostat as set forth in claim 9 wherein the plate-like member is provided with a cut line running around the edge which is complementary to the wedgeshape portion of the insulating member, said out line being spaced outwardly from said edge, and said platelike member further being provided with a pair of cut lines extending along the opposite sides of said opening from the margin of the plate-like member opposite to the edge shaped complementary to the wedge-shaped under surface of the insulating member.

11. A rheostat comprising an insulating base, a resistance element arcuately arranged on said base, means for making an electrical connection to said resistance element, a shaft rotatably carried from said base substantially concentric with said resistance element, an insulating member carried by said shaft and having a pyramidalshaped under surface and a notch on the upper surface disposed substantially diagonally therefrom, movable contact means slidably engageable with said resistance element and including a resilient member having an opening therein substantially complementary to said insulating member and arranged diagonally thereon from said notch to said pyramidal-shaped under surface, engagement of said movable contact means with said resistance element resiliently Wedging said resilient member against said pyramidal-shaped under surface to preclude backlash between said shaft and said movable contact means, and means for making an electrical connection to said movable contact means.

12. A rheostat as set forth in claim 11 wherein the upper notch is provided with diagonal corners and the pyramidal under surface is provided with diagonal faces, said resilient member having internal edges corresponding to said diagonal edges and surfaces, and said diagonal edges and surfaces and the corresponding edges of said resilient member forming the points of support of said resilient member on said insulating member.

13. A rheostat comprising an insulating base, a resistance element arranged on said base, means for making an electrical connection to said resistance element, a settable member carried from said base and movable past said resistance element, an insulating member carried by said settable member and having a wedgeshaped surface, movable contact means slid-ably engageable with said resistance element and including a resilient member having an edge generally complementary to said Wedge-shaped surface of said insulating member and resiliently Wedging against said surface to preclude backlash between said settable member and said movable contact means, and means for making an electrical connection to said movable contact means.

14. A rheostat as set forth in claim 13 wherein the wedge-shaped surface is substantially pyramidal.

15. A resilient contact carrying member for a rheostat or the like comprising a substantially flat plate-like mem ber of resilient material, said plate-like member having an opening provided at front and rear with diagonally disposed surfaces for Wedging engagement with a mounting member, a pair of cut lines extending from the rear margin into said plate on opposite sides of said opening, an additional out line extending around the diagonally disposed surfaces at the front of said opening and spaced outwardly therefrom, said last mentioned out line overlapping the first mentioned pair of cut lines, and means on said plate for holding a contact member.

16. A contact carrying member as set forth in claim 15 wherein the means on said plate for carrying a contact member comprises an extending portion with downwardly directed ears on the opposite sides and extending edge thereof, and a struck out tongue forming With said cars a cup for receiving a contact member, a portion of the material in said cup being displaced into said cup from the surface of said plate-like member and forming with said ears and said tongue a universal joint connection for a contact member.

17. The combination comprising the plate-like member set forth in claim 14 and a mounting member therefor, said mounting member being formed of insulating ma terial and having a pyramidal under surface with diagonally arranged surfaces engageable with the front diagonal edges of the plate-like member, and a shelf near the upper rear portion thereof, said mounting member having rear diagonal surfaces extending upwardly from said shell and engageable with the rear diagonal edges of the plate-like member.

18. A rheostat comprising an insulating base, a resistance element arcuately arranged on said base, means for making an electrical connection to said resistance element, a rotary insulating member carried from said base substantially concentric with said resistance ele ment and having a radially projecting portion with a tapered surface substantially confronting said resistance element, resilient movable contact means slidingly engageable With said resistance element and carried by said insulating rotary member, said contact means engaging said resistance element and having a portion substantially complementary to said tapered surface and thereby being resiliently wedged against the radially projecting portion of said rotary member for precluding backlash between said rotary member and said contact means, and means for making an electrical connection to said movable contact means.

References Cited in the file of this patent FOREIGN PATENTS 659,778 Great Britain Oct, 24, 1951 

